test: changes to use template literal

PR-URL: #15937
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>

Author: joanne-jjb

test/parallel/test-cluster-disconnect-race.js

@@ -24,7 +24,7 @@ if (cluster.isMaster) {
   }));
 
   cluster.on('exit', common.mustCall(function(worker, code) {
-    assert.strictEqual(code, 0, 'worker exited with error');
+    assert.strictEqual(code, 0, `worker exited with error code ${code}`);
   }, 2));
 
   return;